addHex, nodejs

add_hex.js

 function addHex(left /*: string*/, right /*: string*/) /*: string*/ {
 if (left.length < right.length) {
    [left, right] = [right, left];
  }

  // left is "longer"
  // left = a b c
  // right = d
  // --> add c + d,
  // --> carry 1 to b + <nothing>
  // --> ... etc
  let carry = 0;
  const zero = '0';
  const max = 0xf;
  for (let i = 0, leftLen = left.length, rightLen = right.length; i < leftLen; i++) {
    const leftI = leftLen - i - 1;
    const rightI = rightLen - i - 1;
    const atLeft = parseInt(left[leftI], 16);
    const atRight = parseInt(rightI < 0 ? zero : right[rightLen - i - 1], 16);
    let newLeft = atLeft + atRight + carry;
    if (newLeft > max) {
      carry = 1;
      newLeft -= max;
    } else {
      carry = 0;
    }

    left = left.substr(0, leftI) + newLeft.toString(16) + left.substr(leftI + 1);
  }

  if (carry) {
    left = '1' + left;
  }

  return left;
}